In general, TFT liquid crystal panels are constructed by filling liquid crystals between an array side substrate having TFT devices built therein and a color filter substrate. They are based on the active matrix addressing scheme where TFTs apply controlled voltages for controlled alignment of liquid crystals.
In the manufacture of the array side substrate, patterns are formed in plural layers on a mother glass such as non-alkaline glass by repeating light exposure through originals having circuit patterns drawn thereon, known as large-size photomasks. On the other hand, the color filter side substrate is manufactured by a lithographic process known as dye immersion process. In the manufacture of both array and color filter side substrates, large-size photomasks are necessary. For a high accuracy of light exposure, such large-size photomasks are typically made of synthetic quartz glass characterized by a low coefficient of linear expansion.
So far, liquid crystal panels have progressed to higher definitions from VGA to SVGA, XGA, SXGA, UXGA and QXGA. It is believed that degrees of definition ranging from 100 pixels per inch (ppi) class to 200 ppi class are necessary. This, combined with an expanding exposure range, imposes a strict exposure accuracy, especially overlay accuracy, on the TFT array side.
Some panels are manufactured using the technology known as low-temperature polysilicon. In this case, it has been studied to bake a driver circuit or the like on a peripheral portion of glass, aside from the panel pixels, which requires light exposure of higher definition.
For a higher accuracy of light exposure, there is a need for large-size photomask-forming substrates exhibiting a higher flatness in the actual use state, that is, when supported in an exposure apparatus.
While the methods for processing large-size photomask-forming substrates utilize for flatness correction a reaction force against the elastic deformation generated when the substrate itself is forced against the processing platen, there is a drawback that as the substrate size becomes larger, the reaction force considerably decreases, leading to a reduction of the ability to remove moderate irregularities on the substrate surface. As the size of substrates becomes larger, the prior art polishing method is difficult to finish to the desired flatness.
To solve these problems, the inventors proposed in JP-A 2003-292346 and JP-A 2004-359544 a method for improving the flatness of a large-size glass substrate having a diagonal length of at least 500 mm, achieving a parallelism of 50 μm or less and a flatness/diagonal length of 6.0×10−6 or less.
However, to perform multiple pattern panelization through a single exposure for the purpose of increased productivity of panel manufacture, there arises a need for large-size photomask substrates having a diagonal length of 1,000 mm or greater. Glass substrates are required to satisfy both large size and high flatness. In the case of such large-size glass substrates, sometimes a substrate becomes deflected on the actual use attitude where it is held horizontally in an exposure apparatus, failing to acquire the desired flatness. Since the deflection of a substrate by its own weight is in inverse proportion to the cube of its thickness, the size enlargement has a propensity that as the size of a substrate is increased, its thickness is also increased. As a result, the weight of large-size glass substrates is also increased. It is thus desired to have a method of flattening such large-size glass substrates to a higher level of flatness.